The present invention generally relates to scatter measurement in a diagnostic imaging system. In particular, the present invention relates to measurement and correction of scatter using an occluded detector ring in a multiple ring diagnostic imaging system.
Diagnostic imaging systems encompass a variety of imaging modalities, such as x-ray systems, computerized tomography (CT) systems, ultrasound systems, electron beam tomography (EBT) systems, magnetic resonance (MR) systems, and the like. Diagnostic imaging systems generate images of an object, such as a patient, for example, through exposure to an energy source, such as x-rays passing through a patient, for example. The generated images may be used for many purposes. For instance, internal defects in an object may be detected. Additionally, changes in internal structure or alignment may be determined. Fluid flow within an object may also be represented. Furthermore, the image may show the presence or absence of items in an object. The information gained from diagnostic imaging has applications in many fields, including medicine and manufacturing.
EBT scanners are generally described in U.S. Pat. No. 4,352,021 to Boyd, et al. (Sep. 28, 1982), and U.S. Pat. Nos. 4,521,900 (Jun. 4, 1985), 4,521,901 (Jun. 4, 1985), 4,625,150 (Nov. 25, 1986), 4,644,168 (Feb. 17, 1987), 5,193,105 (Mar. 9, 1993), 5,289,519 (Feb. 22, 1994), 5,719,914 (Feb. 17, 1998) and 6,208,711 all to Rand, et al., and U.S. Pat. No. 5,406,479 to Harman (Apr. 11, 1995). The above listed patents are referred to and incorporated herein by reference in their entireties.
As described in the above-referenced patents, an electron beam is produced by an electron source at the upstream end of an evacuated, generally conical shaped housing chamber. A large negative potential (e.g. xe2x88x92130 kV or xe2x88x92140 kV) on a cathode of the electron source accelerates the electron beam downstream along an axis of the housing chamber. Further downstream, a beam optical system that includes solenoid, quadrupole, and deflection coils focus and deflect the beam to scan along an x-ray producing target. EBT systems utilize a high energy beam of electrons to strike the target and produce x-rays for irradiating an object to be imaged. The point where the electrons strike the target is called the xe2x80x9cbeam spotxe2x80x9d. The final beam spot at the target is smaller than that produced at the electron source, and must be suitably sharp and free of aberrations so as not to degrade definition in the image rendered by the scanner.
The x-rays produced by the target penetrate a patient or other object and are detected by an array of detectors. The detector array, like the target, is coaxial with and defines a plane orthogonal to the scanner axis of symmetry. The output from the detector array is digitized, stored, and computer processed to produce a reconstructed x-ray image of a slice of the object, typically an image of a patient""s anatomy such as the heart or lungs.
An EBT scanner allows for the collection of many angles of view and scanning of a number of slices in a short time. There is no mechanically moving gantry. Both high resolution and dynamic scanning modes may be provided while eliminating the need for any target or detector motion by replacing conventional x-ray tubes with electron beam technology.
Multiple views may be generated by magnetically steering a focused electron beam along a 210 degree target ring positioned beneath a subject. Opposite the target ring is a stationary detector ring of Cadmium tungstate crystals encompassing a 216 degree arc above the subject. Photodiodes in the detector ring are used for recording transmitted x-ray intensity. X-ray intensity data may be processed to produce an image.
In order to help ensure that diagnostic images may be used reliably, image correction is advantageous in diagnostic imaging systems. The image correction in diagnostic imaging systems is important for several reasons, including image quality and system performance. Poor image quality may prevent reliable analysis of the image. For example, a decrease in image contrast quality may yield an unreliable image that is not usable clinically. Additionally, the advent of real-time imaging systems has increased the importance of generating clear, high quality images. Inaccuracies or errors in an imaging system may result in blurring, streaking, or introduction of ghost images or artifacts in a resulting image. For example, if electrons are scattered by a bone or other dense component in an object and then impact a detector in the imaging system, artifacts in a scanned image may result. The correction of diagnostic images may help to produce a distinct and usable representation of an object.
Scatter resulting from the diagnostic imaging system or object being imaged creates x-rays that do not original from the x-ray source. Scattered x-rays spuriously increase the signal detected by a detector in an imaging system, particularly if the primary detected signal is highly attenuated (i.e., the detected signal is weakened due to absorption of incident x-rays). When the image is reconstructed with scatter errors, artifacts are present in the image. Artifacts reduce the diagnostic quality of the image.
In order to correct for scatter, a measurement of the scatter is taken and subtracted from the primary signal. Present scatter correction methods subtract the scatter signal from the primary data signal and make many assumptions about the nature of the scatter signal. As a result, significant scatter artifacts remain after scatter correction. Ideally, the scatter would be measured directly so that the scatter could be subtracted without making assumptions.
Thus, a need exists for improving image quality by reducing scatter. A need also exists for a method and apparatus for scatter correction without making assumptions regarding the scattered x-rays.
Certain embodiments include a system and method for scatter measurement and correction. The method includes performing a calibration scan using a phantom to measure a scatter signal ratio between scatter x-rays impacting a first detector ring and scatter x-rays impacting a second detector ring. The scatter signal ratio is used to determine a scatter scale factor. The method further includes positioning the collimator such that the first detector ring is occluded from a path of primary x-rays generated by a target. The method also includes executing a low exposure scan to obtain x-ray scatter data using the first detector ring and applying the scatter scale factor to the scatter data to produce scaled scatter data. The method further includes obtaining image data using the first detector ring and/or the said second detector ring and adjusting the image data using the scaled scatter data.
The phantom may be a water-filled phantom including a bar in an interior portion of the phantom to block x-rays. The image data may be obtained during a 50 ms scan. Additionally, the scaled scatter data may be smoothed using a filter, such as a Gaussian convolution filter. The scaled scatter data may be further scaled for overestimation of x-ray scatter. In a certain embodiment, the adjusting step includes subtracting the scaled scatter data from the image data. In a certain embodiment, the scatter data is obtained during preview scanning by including one full scan for every five preview scans. In another embodiment, scatter data is acquired during a single slice coronary calcium scan.
The system includes first and second detector rings for detecting radiation. The first detector ring is capable of being blocked from a primary path of said radiation. The first detector ring acquires scatter data and at least one of the first detector ring and the second detector ring acquires image data. The system also includes a phantom for use in calibration of a scatter scaling factor for radiation scatter and a processor for reducing scatter in the image data. The processor applies the scatter scaling factor to the scatter data and subtracts the scatter data from the image data.
The system may further include a collimator for controlling the primary path of the radiation. In a certain embodiment, the phantom includes a boundary around an exterior of the phantom, an interior portion of the phantom filled with water, and a bar positioned in the interior portion of the phantom to block radiation. The system may further include a Gaussian convolution filter for smoothing the scatter data. In a certain embodiment, the first and second detector rings include a plurality of detector elements for receiving the radiation. The first and second detector rings may be exposed to a low exposure preview scan to acquire the scatter data. In a certain embodiment, the scatter data is obtained using a series of five low exposure scans and one full scan.